1. Field of the Invention
The field of the invention is that of digital signal transmission systems using phase shift keying.
To be more precise, the present invention concerns a method of and a device for determining the Eb/No ratio of a communication link (for example a microwave link), this ratio representing the ratio of the energy per bit to the noise spectral density, proportional to the bit error rate (BER). The process is used in a modem receiving multiple phase digital signals. The invention can be used to detect loss of carrier recovery.
2. Description of the Prior Art
The ratio of the energy per bit to the noise spectral density, usually called the Eb/No ratio, or the BER is used to assess the quality of transmission, for example the quality of a satellite communication channel. A knowledge of this value enables automatic adaptive correction of the parameters of the demodulator system, for example the bandwidth or the slope of the receive Nyquist filter. It can also be beneficial to display the Eb/No ratio at the receiver to indicate to the user the quality of transmission and the quality of demodulation by their modem.
In existing systems the Eb/No value is deduced from the bit error rate obtained using Viterbi or Reed Solomon type error codecs. However, it takes a long time to determine the BER as this requires integration over a large number of received samples. Also, error coding/decoding can mask the true Eb/No value and the changes made to the filter parameters are then inadequate or incorrect.
One object of the present invention is to overcome these drawbacks.
To be more precise, one object of the invention is to provide a fast and reliable process for determining the Eb/No ratio of a transmission channel.
Another object of the present invention is to provide a process of this kind which is independent of the type of link so that it can be used whether the data transmitted and received is encoded or not. If the data is not encoded the Viterbi or Reed Solomon algorithms are not used and it is then not possible to determine the Eb/No ratio. The invention must make it possible to determine the Eb/No ratio of an uncoded link, for example a differential link.
A further object of the present invention is to detect loss of carrier recovery.
A complementary object of the invention is to provide a device implementing the process defined above.